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User-friendly network access

Anyone can write Swift code to fetch network data, but much harder is knowing how to write code to do it respectfully. In this article we’ll look at building a considerate network stack, taking into account the user’s connection, preferences, and more.

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Optimal network detection

Let’s start with the basics: do we even have an active network connection?

Years ago, before “reachability” was used to make it easier to reach distant parts of an iPhone screen, Apple used the name for a small network class that would detect whether a server could be reached or not. Over the years this has been forked multiple times, and you’ll still commonly see it used in the wild.

These days, though, there’s a much better solution called NWPathMonitor. It comes with the Network framework that was introduced in iOS 12, and we can either restrict it to using particular network types (e.g. WiFi or cellular) or just let it work across all interfaces.

To use it takes a few steps, so we’re going to create a simple wrapper for it so you can either monitor it directly with SwiftUI, or in other frameworks using Combine.

This takes five small steps in total, starting with creating a new class that conforms to the ObservableObject protocol so we can announce updates as they come in. We’ll need to add some properties to this, the most important two of which are one to store the internal NWPathMonitor watching for changes, and one to store a DispatchQueue where the monitoring work will take place.

So, start by adding an import for Network, then add this new class to the same file:

class NetworkMonitor: ObservableObject {
    private let monitor = NWPathMonitor()
    private let queue = DispatchQueue(label: "Monitor")
}

Second, to make this easier to use we’re going to add four properties that describe the network availability we have: whether it’s active (do we have access to the network?), whether it’s expensive (cellular or WiFi using a cellular hotspot), whether it’s constrained (restricted by low data mode), and the exact connection type we have (WiFi, cellular, etc).

So, add these properties to NetworkMonitor:

var isActive = false
var isExpensive = false
var isConstrained = false
var connectionType = NWInterface.InterfaceType.other

Third, we need to give the class an initializer that tells the internal NWPathMonitor what it should do when changes come in, and tells it start monitoring on our private queue:

init() {
    monitor.pathUpdateHandler = { path in
        // more code here
    }

    monitor.start(queue: queue)
}

Now for the fractionally more complex part: whenever that // more code here closure is run, we need to store our latest networking values in our property, then notify anyone who is watching for updates.

This is mostly easy to do, but we do need to make an array of all possible connection types then find the first one that is actually available.

Replace the // more code here comment with this:

self.isActive = path.status == .satisfied
self.isExpensive = path.isExpensive
self.isConstrained = path.isConstrained

let connectionTypes: [NWInterface.InterfaceType] = [.cellular, .wifi, .wiredEthernet]
self.connectionType = connectionTypes.first(where: path.usesInterfaceType) ?? .other

DispatchQueue.main.async {
    self.objectWillChange.send()
}

For the final step, we need to create an instance of NetworkMonitor then inject it into the SwiftUI environment. We can do that in the scene delegate by replacing the let contentView line with this:

let monitor = NetworkMonitor()

let contentView = ContentView().environmentObject(monitor)

And that’s it! We now have a reusable network monitor that works great in SwiftUI, but also can be used in UIKit and other frameworks because it just uses a simple Combine publisher.

To try it out in SwiftUI, use code like this:

struct ContentView: View {
    @EnvironmentObject var network: NetworkMonitor

    var body: some View {
        Text(verbatim: """
        Active: \(network.isActive)
        Expensive: \(network.isExpensive)
        Constrained: \(network.isConstrained)
        """)
    }
}

Once it’s running, you’ll find your UI automatically updates if you toggle WiFi or flight mode.

Waiting for access

If you want really great network access then you need to create your own URLSession. This is easy to do, and although it gives us control over the same constrained and expensive checks as before, we now also gain the ability to control caching and waiting – it’s one of the most beautiful parts of Apple’s APIs, and I wish more folks used it!

Here’s how a basic configuration might look:

func makeRequest() {
    let config = URLSessionConfiguration.default
    config.allowsExpensiveNetworkAccess = false
    config.allowsConstrainedNetworkAccess = false

    let session = URLSession(configuration: config)
    let url = URL(string: "https://www.apple.com")!

    session.dataTask(with: url) { data, response, error in
        print(data)
    }.resume()
}

With expensive and constrained network access disabled, that request will fail if Low Data Mode is enabled or if the user is on an expensive connection – i.e., cellular or WiFi backed by a cellular hotspot.

The nice thing about this approach is that it means all our requests honor our settings automatically – you can stash that session away somewhere, then make requests on it whenever you need.

But the real power is that we get new functionality, including the ability to defer our network request until access becomes available. This means we can start the request for some work now, provide it with whatever functionality we want to execute when it completes, and iOS will simply hold on to that until we finally have network access.

What does it take to make that happen? Just one Boolean:

config.waitsForConnectivity = true

To try it out, add that line to makeRequest(), then rewrite ContentView so it has a button that triggers makeRequest() when tapped:

var body: some View {
    Button("Fetch Data", action: makeRequest)
}

Try running that on a real device that has flight mode on and WiFi off – you’ll see nothing happens. But as soon as you enable some sort of network, you’ll see something like “Optional(67820 bytes)”, which is the completed download of the Apple website.

This is the ideal way of working with data, particularly when it’s large: we want to have it at some point but not necessarily now, and not if the user is using an expensive or constrained network.

But having a custom URL session allows us to go one step further: we can have complete control over the way data is cached. By default we’ll have our data while our app is running, but we can just adjust our configuration so that data is always reloaded like this:

config.requestCachePolicy = .reloadIgnoringLocalCacheData

Putting all this together, we can get sensible caching, requests that wait until network access is available, requests that refuse to work over expensive or constrained networks, and super slick network detection with NWPathMonitor – not bad!

Challenges

There are a few tasks I recommend you try to make our code more flexible:

Update the NetworkMonitor class so that we can create the internal NWPathMonitor using a specific interface type if needed. This should be alongside the current initializer, but it should allow us to request WiFi specifically.
Also add a way to stop our network monitor from working.

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